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Materials 2016, 9(8), 651; doi:10.3390/ma9080651

A Comparison of Classical Force-Fields for Molecular Dynamics Simulations of Lubricants

1
Department of Mechanical Engineering, Imperial College London, South Kensington Campus, Exhibition Road, London SW7 2AZ, UK
2
Shell India Markets Private Limited, 8B RMZ Centennial Building, Kundanahalli Main Road, Bangalore 560048, India
3
Shell Global Solutions UK Ltd., Brabazon House, Manchester M22 0RR, UK
*
Author to whom correspondence should be addressed.
Academic Editor: Magd Abdel Wahab
Received: 22 June 2016 / Revised: 20 July 2016 / Accepted: 28 July 2016 / Published: 2 August 2016
(This article belongs to the Special Issue Numerical Analysis of Tribology Behavior of Materials)
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Abstract

For the successful development and application of lubricants, a full understanding of their complex nanoscale behavior under a wide range of external conditions is required, but this is difficult to obtain experimentally. Nonequilibrium molecular dynamics (NEMD) simulations can be used to yield unique insights into the atomic-scale structure and friction of lubricants and additives; however, the accuracy of the results depend on the chosen force-field. In this study, we demonstrate that the use of an accurate, all-atom force-field is critical in order to; (i) accurately predict important properties of long-chain, linear molecules; and (ii) reproduce experimental friction behavior of multi-component tribological systems. In particular, we focus on n-hexadecane, an important model lubricant with a wide range of industrial applications. Moreover, simulating conditions common in tribological systems, i.e., high temperatures and pressures (HTHP), allows the limits of the selected force-fields to be tested. In the first section, a large number of united-atom and all-atom force-fields are benchmarked in terms of their density and viscosity prediction accuracy of n-hexadecane using equilibrium molecular dynamics (EMD) simulations at ambient and HTHP conditions. Whilst united-atom force-fields accurately reproduce experimental density, the viscosity is significantly under-predicted compared to all-atom force-fields and experiments. Moreover, some all-atom force-fields yield elevated melting points, leading to significant overestimation of both the density and viscosity. In the second section, the most accurate united-atom and all-atom force-field are compared in confined NEMD simulations which probe the structure and friction of stearic acid adsorbed on iron oxide and separated by a thin layer of n-hexadecane. The united-atom force-field provides an accurate representation of the structure of the confined stearic acid film; however, friction coefficients are consistently under-predicted and the friction-coverage and friction-velocity behavior deviates from that observed using all-atom force-fields and experimentally. This has important implications regarding force-field selection for NEMD simulations of systems containing long-chain, linear molecules; specifically, it is recommended that accurate all-atom potentials, such as L-OPLS-AA, are employed. View Full-Text
Keywords: tribology; molecular dynamics; force-fields; lubricants tribology; molecular dynamics; force-fields; lubricants
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MDPI and ACS Style

Ewen, J.P.; Gattinoni, C.; Thakkar, F.M.; Morgan, N.; Spikes, H.A.; Dini, D. A Comparison of Classical Force-Fields for Molecular Dynamics Simulations of Lubricants. Materials 2016, 9, 651.

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